Background

Variations in Atlantic Ocean sea surface temperatures alternate between warm and cool phases about every 60-68 years. This natural seasaw in ocean temperatures is called the Atlantic Multidecadal Oscillation. Atlantic Ocean temperature variations are caused by large-scale circulation of heat in the ocean. An important system of heat carrying currents in the Atlantic Ocean is called the Atlantic Meridional Overturning Circulation, or AMOC. The AMOC is a principle component of global thermohaline (temperature and salinity) circulation that helps to regulate climate and temperature at low and high latitudes (Figure 1).

Due to the absence of long-term observations of ocean circulation and heat transport, there have been limited direct studies to support the hypothesis that ocean circulation helps to drive long-term variability in the ocean (i.e. Atlantic Multidecadal Oscillation). A new study led by Gerard McCarthy of the University of Southampton uses sea-level observations from tide gauges along the east coast of the US to examine ocean circulation anomalies, and compares the fluctuations in sea level with large-scale temperature variations in the Atlantic Ocean.

Methods

This study examines the gradient in sea level along the US east coast to diagnose Atlantic Ocean circulation. Sea level variability between Florida and Boston can be divided into two separate regions. From Cape Hatteras to Boston (north), sea level is most similar to the overturning circulation in the North Atlantic (e.g. AMOC). From Florida to Cape Hatteras (south), Gulf Stream variability is more strongly associated with sea level fluctuations. Cape Hatteras serves as a theoretical boundary between subtropical (southern) and subpolar (northern) ocean gyres in the North Atlantic (Figure 2). An ocean gyre is a system of currents created by wind patterns and Earth’s rotational force.

Figure 2. Subpolar and subtropical gyres in the North Atlantic. Arrow show approximate location of Cape Hatteras, North Carolina where the two circulation systems are split based on sea level variability (de Boer, 2010).

McCarthy and co-authors developed a circulation index by differencing subtracting the average subtropical sea level from subpolar gyre sea-level using data from tidal gauges. When this index is positive, ocean circulation, and therefore heat transport, has a stronger northward flow and northerly extent. Since long-term temperature variability is most pronounced in the subpolar region, the circulation index can be used to examine fluctuations in both heat content changes (inferred from sea level) and the Atlantic Multidecadal Oscillation since 1960.

Findings

The observed sea-level variability and circulation index supports the hypothesis that ocean circulation changes and heat transports in the North Atlantic influence fluctuations in the Atlantic Multidecadal Oscillation.

Increased ocean heat coincides with the positive phase of the Atlantic Multidecadal Oscillation. This highlights ocean circulation as an important component to decadal variability of sea surface temperatures in the North Atlantic Ocean. Contribution from atmospheric variability known as the North Atlantic Oscillation also influences ocean circulation, and on decadal timescales influences the Atlantic Multidecadal Oscillation through air-sea heat exchanges. The North Atlantic Oscillation modulates heat transport, sea surface temperatures and ultimately the warm/cool phase of the Atlantic Multidecadal Oscillation (Figure 3).

The circulation index leads the rate of ocean heat change by 2 years. This provides an early indication of northern temperature change. Therefore, sea-level differences between northern and southern coastal regions signal ocean circulation changes that precede phase changes in the Atlantic Multidecadal Oscillation.

Significance

Since the coastal tidal gauge network formed in the early 1920s, the Atlantic Multidecadal Oscillation has entered a warm phase in the 1920s and mid-1990s, and a cool phase in the 1960s. The circulation index presented in this study suggests that the Atlantic Multidecadal Oscillation is now transitioning to a cool phase with a reduction in overturning circulation and heat content. As the warm Atlantic Multidecadal Oscillation transitions to the cool phase, sea-level will begin to rise north of Cape Hatteras.

Overturning circulation in the Atlantic is slowing down and is reinforcing cooler sea surface temperatures in the North Atlantic. While this may offer temporary relief to rising ocean temperatures, the coupling between these processes raises sea-level heights along the northeast coast of the US, putting major coastal cities like Boston and New York at elevated coastal flooding risks.